Definitely Predator. A large part of it is sentimental because when I was young I saw the first Predator and fell in love with it and then the sequel, etc. I never saw any of the Alien films til much later and had already developed my Pred obsession.
I just think the Predator is a cooler character, aesthetically and in concept. Aliens look way cool, but they also just seem like a big bug kinda creature and to me they don't have as much individuality as a Pred. To me, a Pred is more of a warrior and more calculating, more honorable. They have a code and technology and all kinds of wicked gadgets. In a way, they're more humanized, but that adds to the appeal. Plus, they're just cooler looking to me too.